Determine depth of a pixel via PointCloud2

edit

Hello,

I subscribe to a Point Cloud2 message.

Here is my callback:

void callback_depth_image_d435 (const sensor_msgs::msg::PointCloud2::SharedPtr point_cloud2_msgs)
{
  //sensor_msgs::msg::PointCloud2& cloud = ;
  pcl::PointCloud<pcl::PointXYZ> point_cloud;

  // convert point clouds2 msgs data to points
  pcl::fromROSMsg(*point_cloud2_msgs, point_cloud);

  // resize vector every callback
  point_cloud2_points.resize(point_cloud.points.size());


  for(unsigned long i = 0; i < point_cloud.points.size(); i++)
  {
    // get points
    point_cloud2_points.at(i).x = point_cloud.points.at(i).x;
    point_cloud2_points.at(i).y = point_cloud.points.at(i).y;
    point_cloud2_points.at(i).z = point_cloud.points.at(i).z;       
  }
}

I need one point for each pixel of my camera image for my object detection. I have set my camera image to 480 x 640. Now I actually expected to get a point for each pixel with the following function.

   pcl::fromROSMsg(*point_cloud2_msgs, point_cloud);

but since the size of the "point_cloud" is constantly changing, this means that I constantly have a different number of pixels?

Can anyone explain the reason for this?

Thanks in advance